1. Field of the Invention
The present invention relates generally to imaging systems and, more particularly, to systems and methods for creating stable camera optics. In addition, the preferred embodiments provide a system and method for creating stable optics in medical imaging systems having optical couplings prone to discoloration by local contaminants.
2. Discussion of the Background
A variety of medical imaging systems are known. Some illustrative imaging systems include nuclear medical imaging systems (e.g., gamma cameras), computed tomography (CT or CAT) systems, magnetic resonance imaging (MRI) systems, positron-emission tomography (PET) systems, ultrasound systems and/or the like.
With respect to nuclear medical imaging systems, nuclear medicine is a unique medical specialty wherein radiation (e.g., gamma radiation) is used to acquire images that show, e.g., the function and/or anatomy of organs, bones and/or tissues of the body. Typically, radioactive compounds, called radiopharmaceuticals or tracers, are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. These radiopharmaceuticals produce gamma photon emissions that emanate from the body and are captured by a scintillation crystal, with which the photons interact to produce flashes of light or “events.” These events can be detected by, e.g., an array of photo-detectors, such as photomultiplier tubes, and their spatial locations or positions can be calculated and stored. In this manner, an image of an organ, tissue or the like under study can be created from the detection of the distribution of the radioisotopes in the body.
A number of illustrative imaging systems are shown in the following United States Patents and Publications, the entireties of which are incorporated herein by reference: (1) U.S. Pat. No. 5,059,798, issued on Oct. 22, 1991, entitled Frangible Bonding of Photomultiplier Tubes for Use In Scintillation Cameras and PET Scanners, listed as assigned to Siemens Gammasonics, Inc.; (2) U.S. Pat. No. 4,605,856, issued on Aug. 12, 1986, entitled Method and Device for Stabilizing Photomultiplier Tubes of Radiation Imaging Device Against Drift, also listed as assigned to Siemens Gammasonics, Inc.; (3) U.S. Pat. No. 4,574,478, issued on Mar. 11, 1986, entitled Method and Device for Demounting In a Radiation Detector a Photomultiplier Tube; (4) U.S. Published Patent Application No. 20040036026, published on Feb. 26, 2004, and filed on Aug. 21, 2002, naming inventors J. Engdahl, et al., and entitled System and Method for Calibrating and Tuning a Gamma Camera, assigned to the present assignee; and (5) U.S. Published Patent Application No. 20030034455A1, published on Feb. 20, 2003, entitled Scintillation Detector System and Method Providing Energy and Position Information, filed on Apr. 3, 2002, which states, inter alia, in paragraph [0099]:
“A preferred reflector 70 is made of a material whose reflectivity will not be degraded by a significant amount by wetting with materials used to provide an optical coupling between the light sensors and the optical window or between the optical window and the scintillator, such as optical greases, adhesives and potting compounds. Conventional reflective materials, such as porous teflon, lose their reflectivity in these situations. A preferred reflector is one whose reflectivity does not degrade by more than about 20% when wetted by the optical coupling material used at the interface where the reflector is installed (or when exposed to a potting material in general), and preferably one that does not degrade by more than about 10%. A particularly preferred reflector is a white polyester film, such as Lumirror™ polyester film sold by Toray Industries, Inc. previously sold for use as a reflector plate for LCD back-lighting applications.”
FIG. 1 depicts components of a typical nuclear medical imaging system 100 (i.e., having a gamma camera or a scintillation camera) which includes a gantry 102 supporting one or more detectors 108 enclosed within a metal housing and movably supported proximate a patient 106 located on a patient support (e.g., pallet) 104. Typically, the positions of the detectors 108 can be changed to a variety of orientations to obtain images of a patient's body from various directions. In many instances, a data acquisition console 200 (e.g., with a user interface and/or display) is located proximate a patient during use for a technologist 107 to manipulate during data acquisition. In addition to the data acquisition console 200, images are often developed via a processing computer system which is operated at another image processing computer console including, e.g., an operator interface and a display, which may often be located in another room, to develop images. By way of example, the image acquisition data may, in some instances, be transmitted to the processing computer system after acquisition using the acquisition console.
More specifically, gamma cameras typically use a scintillating material such as, e.g., thallium iodide doped sodium iodide (Nal(TI)) to interact with gamma rays, creating photons, which must find their way out of the Nal(TI) and into a photomultiplier tube (PMT). Typically, there is at least one physical interface between the Nal(TI) and the PMT. Because the index of refraction of most scintillation crystals is substantially higher than 1.0, getting the scintillation light out of the crystal and into a PMT typically involves the use of an interface including an optical coupling medium.
This interface usually includes a material with an appropriate refractive index (RI) that will allow as many photons as possible to pass to the PMT, regardless of incident angle of the photon to the exiting surface of the Nal(TI). This material with an appropriate RI is typically a silicone-based material. Usually, silicone is chosen because of its RI and because of its stability over time. However, silicone-based materials have drawbacks that compromise the optics. For example, many silicone-based materials used are liquid-based “greases”. These materials are typically very difficult to work with. They tend to migrate away from the area(s) where they are needed, and they tend to pick up contaminants and to discolor. Despite the drawbacks that these greases tend to pick up contaminants from the local environment and to discolor because of the absorbed contaminants, most gamma cameras use a silicone-based optical grease. Another drawback of these greases is that they also require extensive mechanical devices in order to hold the PMTs in the grease. In addition, these greases need to be replaced in the field relatively frequently (such as, e.g., about every 2-5 years) because of discoloration, which discoloration leads to signal degradation and to poor gamma camera performance. Accordingly, this discoloration causes the need for costly and time consuming on-site repairs. Most grease-based gamma cameras require a complete rebuild of its optics every 2-5 years due to these problems. As should be appreciated, this creates a significant amount of downtime for the facilities using these devices, which can be not only very time consuming, but very costly.
An alternative to these silicon-based greases has been the use of silicone-based “gels.” These gels are “mechanically” much more forgiving than the above-noted greases. Among other things, these silicon-based gels are usually semi-solid and generally do not migrate. As a result, these gels can essentially act as a mechanical device to hold the PMTs in place. These gels will also tend to absorb less contamination than the greases. However, the present inventors have discovered that these silicone gels are still rather prone to discolor because of interactions of the platinum catalyst typically used in these gels with local contaminants (such as, e.g., outgassing epoxies from electronic components, coatings from parts within the gamma camera [e.g., organic coatings in contact with the gel], plasticizers from wiring inside the gamma camera and/or the like). Thus, as with silicon greases, the discoloration of silicon gels leads to degraded optics, which similarly triggers the need for on-site repairs and actions, and in severe cases can require a complete rebuilding of the gamma camera optics.
The present inventors have found that one noteworthy contaminant is the optical coating often used to help direct photons into the PMTs. This coating is typically placed onto the physical device (e.g., glass or plastic) that holds the PMTs. Because most PMTs have a geometry that does not allow 100% area coverage of the optical interface, some photons would be lost (e.g., in areas between the PMTs, leading to increased scan times or poorer images. Accordingly, this coating is typically used to reflect the photons back into an area that will allow them to be captured by a PMT for data processing.
While there has been some technical progress in striving to overcome some of the field repair problems, including the development of complex mathematical corrections to try to account for optical degradations, there remains a continued need for further improvements. Thus, while a variety of systems and methods are known, there remains a continued need for improved systems and methods overcoming the above and/or other problems with existing systems and methods.